(1) Field of the Invention
This invention relates to MR imaging methods and apparatus using NMR (nuclear magnetic resonance) phenomenon, and more particularly to an MR imaging method and apparatus for effecting high-speed imaging based on GRASE (gradient and spin echo) technique.
(2) Description of the Related Art
An MR imaging apparatus is known which effects a pulse sequence for high-speed imaging called GRASE technique (U.S. Pat. No. 5,270,654, and K. Oshio and D. A. Feiberg "GRASE (Gradient and Spin Echo) Imaging: A Novel Fast MRI Technique", Magnetic Resonance in Medicine 20, 344-349, 1991). The pulse sequence based on the GRASE technique is one combining the EPI (Echo Planar Imaging) technique which generates gradient echo signals by switching the polarity of a gradient magnetic field, and the RARE (Rapid Acquisition with Relaxation Enhancement) which generates spin echo signals by using an excitation RF (Radio Frequency) pulse (also called 90.degree. pulse since it rotates the spin phase of protons 90.degree.) and refocus RF pulses (also called 180.degree. pulses since they rotate the spin phase of protons 180.degree.).
In this method, a group of data acquired from a plurality of echo signals generated are arranged in the same k space (also called a raw data space), and an image is reconstructed by a two-dimensional Fourier transform of this k space. This method can provide a T2-enhanced image at high speed by generating a plurality of echo signals (there being two types; gradient echo signals and spin echo signals) within a period of one pulse sequence (i.e. repetition time) to reduce the number of cycles within the repetition time. The "T2" in the T2-enhanced image refers to a transverse relaxation time (also called spin-spin relaxation time). The T2-enhanced image is one emphasizing water in the body of an examinee.
The conventional GRASE technique noted above indeed enables high-speed imaging. However, since a group of data acquired from a plurality of echo signals (each including different information) are arranged in a single k space, a T2-enhanced image can be reconstructed from the k space only with one contrast.